1. Field of the Invention
The present invention relates to an apparatus for controlling the air-fuel ratio of an exhaust gas emitted from an internal combustion engine, and more particularly to an apparatus for controlling the air-fuel ratio of an exhaust gas that is purified by a catalytic converter of the nitrogen-oxide-absorption type that is disposed in the exhaust passage of an internal combustion engine.
2. Description of the Related Art
The applicant of the present application has proposed a technique for controlling the air-fuel ratio of an exhaust gas that enters a catalytic converter, or more specifically the air-fuel ratio of a combusted air-fuel mixture which, when burned, enters as an exhaust gas into a catalytic converter and is recognized as the concentration of oxygen in the exhaust gas, as disclosed in Japanese laid-open patent publication No. 11-93740, for example.
According to the disclosed system, an exhaust gas sensor (O2 sensor) for detecting the concentration of a certain component, e.g., oxygen, of the exhaust gas that has passed through the catalytic converter is disposed downstream of the catalytic converter, and the air-fuel ratio of the exhaust gas that enters the catalytic converter is controlled depending on the output of the exhaust gas sensor, i.e., the detected value of the concentration of oxygen.
Specifically, the purifying capability of a catalytic converter, i.e., the ability of a catalytic converter to purify NOx (nitrogen oxide), HC (hydrocarbon), CO (carbon monoxide), etc. is optimum irrespectively of the deteriorated state of the catalytic converter when the air-fuel ratio of the exhaust gas that enters the catalytic converter is close to a stoichiometric air-fuel ratio and the output of the O2 sensor as the exhaust gas sensor is settled to a certain output value. According to the above proposed technique, therefore, the certain output value is used as a target value for the output of the O2 sensor, and the air-fuel ratio of the exhaust gas that enters the catalytic converter is controlled according to a feedback control process in order to converge the output of the O2 sensor to the target value.
An exhaust system ranging from an upstream side of the catalytic converter to the O2 sensor disposed downstream of the catalytic converter, i.e., a system for generating the output of the O2 sensor from the air-fuel ratio of the exhaust gas that enters the catalytic converter, generally has a relatively long dead time owing to the catalytic converter included in the exhaust system. Stated otherwise, when the air-fuel ratio of the exhaust gas that enters the catalytic converter is changed, a relatively long dead time is required until the output of the O2 sensor reflects the change in the air-fuel ratio. According to the above proposed technique, data representing an estimated value of the output of the O2 sensor after the dead time of the exhaust system is sequentially determined. Then, a manipulated variable defining an air-fuel ratio for the exhaust gas entering the catalytic converter, i.e., a target air-fuel ratio for the exhaust gas, is sequentially generated in order to converge the estimated value of the output of the O2 sensor which is represented by the above data to the target value, and the air-fuel ratio of an air-fuel mixture actually combusted by the internal combustion engine is manipulated depending on the target air-fuel ratio. In this manner, the effect of the dead time is compensated for, and the control process for converging the output of the O2 sensor to the target value is stably carried out.
Some generally known internal combustion engines mounted on automobiles or the like, i.e., so-called lean-burn engines, are operated such that the air-fuel ratio of an air-fuel mixture combusted by the internal combustion engine and hence the air-fuel ratio of an exhaust gas entering a catalytic converter are controlled at a lean air-fuel ratio, which represents less fuel than at the stoichiometric air-fuel ratio, depending on operating conditions (rotational speed, intake pressure, demanded load, etc.) of the internal combustion engine in order to reduce the fuel consumption and also minimize the amount (absolute amount) of harmful gases contained in the exhaust gas.
While the internal combustion engine is being operated to control the air-fuel ratio at the lean air-fuel ratio, however, it is not possible to control the air-fuel ratio of the exhaust gas that enters the catalytic converter in order to converge the output of the O2 sensor disposed downstream of the catalytic converter to the target value according to the above proposed technique. Under some operating conditions of the internal combustion engine, it is not possible or not preferable to operate the internal combustion engine to control the air-fuel ratio at the lean air-fuel ratio.
If the above proposed technique for achieving the optimum purifying capability of the catalytic converter is applied to the above internal combustion engine, then the internal combustion engine is operated in different modes including an operation. mode (hereinafter referred to as xe2x80x9cstoichiometric operation modexe2x80x9d) in which the air-fuel ratio of the exhaust gas that enters the catalytic converter is controlled at an air-fuel ratio close to the stoichiometric air-fuel ratio in order to converge the output of the O2 sensor disposed downstream of the catalytic converter to the target value, and an operation mode (hereinafter referred to as xe2x80x9clean operation modexe2x80x9d) in which the air-fuel ratio of the exhaust gas that enters the catalytic converter is controlled at a lean air-fuel ratio. Control processes of these operation modes are selectively carried out depending on operating conditions of the internal combustion engine.
While an internal combustion engine is operating in a lean operation mode, the amount of NOx contained in the exhaust gas emitted from the internal combustion engine is generally relatively large. Therefore, the internal combustion engine is combined with an NOx-absorption catalytic converter.
The NOx-absorption catalytic converter comprises a three-way catalyst and an NOx absorbent. NOx absorbents that are available includes an occlusion-type NOx absorbent for occluding NOx when the air-fuel ratio of the exhaust gas entering the catalytic converter is a lean air-fuel ratio and the oxygen concentration in the exhaust gas is relatively high, i.e., NOx in the exhaust gas is relatively high, and an adsorption-type NOx absorbent for adsorbing NOx in the exhaust gas when the air-fuel ratio of the exhaust gas entering the catalytic converter is a lean air-fuel ratio. Irrespectively of whether it is of the occlusion type or the adsorption type, an NOx adsorbent reduces NOx that has been absorbed (occluded or adsorbed) at the lean air-fuel ratio when the air-fuel ratio of the exhaust gas that enters the catalytic converter is a stoichiometric air-fuel ratio or a rich air-fuel ratio (at which the fuel is more than at the stoichiometric air-fuel ratio) and the oxygen concentration in the exhaust gas is relatively low.
More specifically, when the air-fuel ratio of the exhaust gas that enters the catalytic converter becomes a stoichiometric air-fuel ratio or a rich air-fuel ratio, the occlusion-type NOx absorbent discharges the occluded NOx, and the discharged NOx is reduced by a reducing agent such as CO, H2, or the like in the exhaust gas. When the air-fuel ratio of the exhaust gas that enters the catalytic converter becomes a stoichiometric air-fuel ratio or a rich air-fuel ratio, the adsorbed NOx in the adsorption-type NOx absorbent is reduced by the reducing agent in the exhaust gas, and the reduced nitrogen gas is discharged from the NOx absorbent.
The occlusion-type NOx absorbent comprises barium oxide (BaO), and the adsorption-type NOx absorbent comprises sodium (Na), titanium (Ti), or strontium (Sr).
When the internal combustion engine with the NOx-absorption catalytic converter in the exhaust passage is operating in the lean operation mode, the amount of NOx that can be absorbed by the NOx absorbent is limited. Therefore, after the internal combustion engine has operated for a certain period of time, it is necessary to interrupt the lean operation mode and reduce NOx that has been absorbed by the catalytic converter. For example, as disclosed in Japanese laid-open patent publication No. 11-62562, if the absorption of NOx in the catalytic converter is saturated, then the air-fuel ratio is temporarily controlled at a rich air-fuel ratio, and NOx that has been absorbed by the catalytic converter is reduced.
If the internal combustion engine is operated selectively in the lean operation mode and the stoichiometric operation mode, then the internal combustion engine is operated in stoichiometric operation mode and thereafter in the lean operation mode for thereby reducing NOx that has been absorbed by the catalytic converter. That is, during the lean operation mode, the output of the O2 sensor disposed downstream of the catalytic converter represents a leaner air-fuel ratio than the target value in the stoichiometric operation mode. Therefore, when the internal combustion engine switches from the lean operation mode to the stoichiometric operation mode and the process of controlling the air-fuel ratio of the exhaust gas that enters the catalytic converter in order to converge the output of the O2 sensor to the target value is started, the air-fuel ratio of the exhaust gas is controlled at a rich air-fuel ratio immediately after the control process has been started. The catalytic converter can thus reduce NOx.
The catalytic converter can also reduce NOx by positively controlling the air-fuel ratio of the exhaust gas that enters the catalytic converter at a rich air-fuel ratio, as disclosed in Japanese laid-open patent publication No. 11-62562. However, such an arrangement makes the control of operation of the internal combustion engine complex because another dedicated control process separate from the control process of the stoichiometric operation mode is needed.
Under conditions in which the internal combustion engine can be operated in the lean operation mode, it is desirable to provide as many opportunities as possible for performing the control process of the lean operation mode so as to minimize the fuel consumption by the internal combustion engine. To meet such a demand, when it is necessary to interrupt the lean operation mode and perform the stoichiometric operation mode for the reduction of NOx in the catalytic converter, the period of operation of the internal combustion engine in the internal combustion engine should preferably be limited to a period that is only necessary.
When the reduction of NOx in the catalytic converter in the stoichiometric operation mode is completed, the output of the O2 sensor disposed downstream of the catalytic converter changes from an output value corresponding to a lean air-fuel ratio an output value corresponding to a rich air-fuel ratio. Therefore, it is possible to recognize the time when the reduction of NOx in the catalytic converter is completed by detecting the change in the output of the O2 sensor. The inventors of the present invention have attempted to limit a period in which the lean operation mode is interrupted (inhibited) for reducing NOx to a period until the above change in the output of the O2 sensor disposed downstream of the catalytic converter is detected.
However, as described above, the exhaust system including the catalytic converter has a relatively long dead time. Consequently, the above change in the output of the O2 sensor is caused by the control in the stoichiometric operation mode of the air-fuel ratio of the exhaust gas upstream of the catalytic converter up to a time prior to the dead time. Therefore, the control process of the stoichiometric operation mode in a period between the time when the change in the output of the O2 sensor is detected and the time which is earlier than the above time by the dead time, is not necessary for reducing NOx in the catalytic converter. Stated otherwise, for reducing NOx, the lean operation mode is interrupted and the stoichiometric operation mode is performed for the unnecessarily long period of time. The unnecessarily long period of time presents an obstacle to an effort to reduce the fuel consumption by the internal combustion engine and the amount of harmful gases contained in the exhaust gas.
The NOx absorbent of the NOx-absorption catalytic converter is gradually deteriorated as the internal combustion engine is operated for a longer period of time, and as the deterioration of the NOx absorbent progresses, the amount of NOx that can be absorbed thereby in the lean operation mode is reduced. Therefore, when the catalytic converter is deteriorated to a certain degree, it is desirable to evaluate the deteriorated state of the catalytic converter for replacing the catalytic converter or otherwise treating the catalytic converter. The inventors have attempted to determine an integrated amount (or an equivalent thereof) of reducing agents (HC, CO, H2, etc.) for NOx that are given via the exhaust gas to the catalytic converter after the reduction of NOx in the stoichiometric operation mode is started until the above change in the output of the O2 sensor disposed downstream of the catalytic converter is detected, i.e., until the reduction of NOx in the catalytic converter is completed, and evaluate the deteriorated state of the catalytic converter based on the determined integrated amount.
However, because the reducing agent in the exhaust gas given to the catalytic converter in the control process of the stoichiometric operation mode during the period between the time when the change in the output of the O2 sensor is detected and the time which is earlier than the above time by the dead time does not substantially contribute to the reduction of NOx, it has been difficult to appropriately evaluate the deteriorated state of the catalytic converter.
It is therefore an object of the present invention to provide an apparatus for controlling the air-fuel ratio of an exhaust gas emitted from an internal combustion engine, the apparatus being capable of limiting a period in which NOx absorbed by an NOx-adsorption catalytic converter is reduced during operation of the internal combustion engine in a lean operation mode to a short period that is necessary for thereby providing as many opportunities as possible for operating the internal combustion engine in the lean operation mode and hence further reducing the fuel consumption by the internal combustion engine and the amount of harmful gases contained in an exhaust gas emitted from the internal combustion engine.
Another object of the present invention is to provide an apparatus for controlling the air-fuel ratio of an exhaust gas emitted from an internal combustion engine, the apparatus being capable of appropriately evaluating the deteriorated state of a catalytic converter.
To achieve the above objects, there is provided in accordance with the present invention an apparatus for controlling the air-fuel ratio of an exhaust gas from an internal combustion engine, comprising a catalytic converter disposed in an exhaust passage of the internal combustion engine, for absorbing a nitrogen oxide in the exhaust gas when the air-fuel ratio of the exhaust gas flowing from an upstream side into the catalytic converter is a lean air-fuel ratio, and reducing the absorbed nitrogen oxide with a reducing agent in the exhaust gas when the air-fuel ratio of the exhaust gas is a stoichiometric air-fuel ratio or a rich air-fuel ratio, an exhaust gas sensor disposed downstream of the catalytic converter for detecting the concentration of a particular component in the exhaust gas which has passed through the catalytic converter, estimating means for sequentially generating data representing an estimated value of an output of the exhaust gas sensor after a dead time of an exhaust system which ranges from the upstream side of the catalytic converter to the exhaust gas sensor and includes the catalytic converter, control means for using a predetermined output value of the exhaust gas sensor when the air-fuel ratio of the exhaust gas entering the catalytic converter is close to the stoichiometric air-fuel ratio, as a target value for the output of the exhaust gas sensor, and selectively executing a control process in a stoichiometric operation mode for controlling the air-fuel ratio of the exhaust gas entering the catalytic converter in order to converge the estimated value, represented by the data generated by the estimating means, of the output of the exhaust gas sensor to the target value and a control process in a lean operation mode for controlling the air-fuel ratio of the exhaust gas entering the catalytic converter at the lean air-fuel ratio, the arrangement being such that the control means executes the control process in the stoichiometric operation mode after executing the control process in the lean operation mode to perform a reducing process to reduce the nitrogen oxide in the catalytic converter, and reduced-state recognizing means for sequentially recognizing a reduced state of the nitrogen oxide in the catalytic converter based on data generated by the estimating means while the control process in the stoichiometric operation mode is being executed in the reducing process, the control means comprising means for determining whether to switch from the control process in the stoichiometric operation mode to the control process in the lean operation mode or not depending on the reduced state recognized by the reduced-state recognizing means.
With the above arrangement, the control process in the stoichiometric operation mode is carried out in the process of reducing the nitrogen oxide (NOx) absorbed by the catalytic converter while the control process in the lean operation mode is being performed by the control means. Specifically, the air-fuel ratio of the exhaust gas entering the catalytic converter is controlled in order to converge the estimated value, represented by the data generated by the estimating means, of the output of the exhaust gas sensor to the target value, and as a result converge the output of the exhaust gas sensor to the target value. At this time, the control process in the stoichiometric operation mode finally controls the air-fuel ratio of the exhaust gas entering the catalytic converter (hereinafter also referred to as xe2x80x9cupstream-of-catalytic-converter air-fuel ratioxe2x80x9d) at an air-fuel ratio in the vicinity of a stoichiometric air-fuel ratio. However, in an initial stage immediately after the control process in the stoichiometric operation mode has begun, the upstream-of-catalytic-converter air-fuel ratio is basically controlled at a rich air-fuel ratio due to the effect of the lean operation mode that was executed prior to the control process in the stoichiometric operation mode. When the up-stream-of-catalytic-converter air-fuel ratio is controlled in this fashion, NOx in the catalytic converter is reduced by reducing agents which are HC, CO, H2, etc. contained in the exhaust gas.
While the control process in the stoichiometric operation mode is being performed, the reduced-state recognizing means sequentially recognizes the reduced state of NOx in the catalytic converter based on the data generated by the estimating means. The control means then determines whether to switch from the control process in the stoichiometric operation mode to the control process in the lean operation mode based on the reduced state recognized by the reduced-state recognizing means.
The data sequentially generated by the estimating means comprises data representing the estimated (expected) value of the output of the exhaust gas sensor after the dead time of the exhaust system including the catalytic converter, i.e., a system for generating the output of the exhaust gas sensor from the upstream-of-catalytic-converter air-fuel ratio controlled by the control means. Therefore, the reduced state of NOx which is sequentially recognized by the reduced-state recognizing means based on the above data is a reduced state in the future after the dead time. More specifically, at each point of time during the control process in the stoichiometric operation mode, the reduced state of NOx in the future after the dead time is determined as a result of the stoichiometric operation mode already performed up to the point of time, and the reduced state in the future is recognized as an estimated state by the reduced-state recognizing means.
By determining whether to switch from the control process in the stoichiometric operation mode to the control process in the lean operation mode depending on the reduced state thus recognized, the control means can switch from the control process in the stoichiometric operation mode to the control process in the lean operation mode before the reduced state becomes a desired reduced state.
As a consequence, the period for performing the control process in the stoichiometric operation mode to reduce NOx in the catalytic converter, i.e., the period for inhibiting the control process in the lean operation mode, is limited to a necessary period only, making it possible to provide many opportunities for performing the control process in the lean operation mode.
While the exhaust gas sensor preferably comprises an O2 sensor (oxygen concentration sensor), it may also comprise an NOx sensor, i.e., a sensor for detecting the concentration of nitrogen oxygen. If an O2 sensor is used as the exhaust gas sensor, then the target value should preferably comprise a certain constant value in order to achieve the purifying capability of the catalytic converter in the stoichiometric operation mode. If an NOx sensor is used as the exhaust gas sensor, then an output value of the NOx sensor for allowing the catalytic converter to provide a good NOx purifying capability may be established as a target value for the output of the NOx sensor.
The reduced state recognized by the reduced-state recognizing means represents a state in which the reduction of the nitrogen oxide in the catalytic converter is completed after the dead time of the exhaust system, and the control means comprises means for inhibiting the control process in the stoichiometric operation mode from switching to the control process in the lean operation mode until the reduced-state recognizing means recognizes the state in which the reduction of the nitrogen oxide in the catalytic converter is completed after the dead time of the exhaust system.
When at a certain time in the stoichiometric operation mode performed for reducing NOx it is recognized that the reduction of NOx in the catalytic converter is completed after the dead time of the exhaust system, the reduction of NOx in the catalytic converter is basically completed after the dead time from the recognized time even though the air-fuel ratio of the exhaust gas entering the catalytic converter is controlled in any way after the recognized time. After the time when the completion of the reduction of NOx is recognized, therefore, it is not necessary to perform the control process in the stoichiometric operation mode for the reduction of NOx. If operating conditions (rotational speed, intake pressure, demanded load, etc.) of the internal combustion engine are those for performing the control process in the lean operation mode, then the control process in the lean operation mode can be performed without fail. According to the present invention, therefore, the control means inhibits the control process in the stoichiometric operation mode from switching to the control process in the lean operation mode until the reduced-state recognizing means recognizes the state in which the reduction of NOx in the catalytic converter is completed after the dead time of the exhaust system. After the completion of the reduction of NOx is recognized, the control process in the lean operation mode can be carried out depending on the operating conditions of the internal combustion engine. As a result, under the operating conditions capable of performing the control process in the lean operation mode, it can be resumed before the reduction of NOx in the catalytic converter is actually completed.
Therefore, the state in which the control process in the stoichiometric operation mode is performed to reduce NOx in the catalytic converter can be limited to a necessary period, providing many opportunities for carrying out the control process in the lean operation mode. As a result, the fuel consumption by the internal combustion engine can further be reduced.
The reduced-state recognizing means may comprise means for recognizing the state in which the reduction of the nitrogen oxide in the catalytic converter is completed after the dead time of the exhaust system, by comparing the estimated value, represented by the data generated by the estimating means, of the output of the exhaust gas sensor with a predetermined threshold value. The predetermined threshold value represents the output value (e.g., a value identical to the target value) of the exhaust gas sensor at the time the air-fuel ratio of the exhaust gas is an air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio.
For sequentially recognizing the completion of the reduction of NOx after the dead time, the apparatus preferably further comprises reducing agent amount data generating means for generating data representing an integrated amount of the reducing agent given to the catalytic converter until the reduced-state recognizing means recognizes the state in which the reduction of the nitrogen oxide in the catalytic converter is completed after the dead time of the exhaust system after the control process in the stoichiometric operation mode is started, while the control process in the stoichiometric operation mode is being executed in the reducing process, and catalytic converter deterioration evaluating means for evaluating a deteriorated state of the catalytic converter based on the data generated by the reducing agent amount data generating means.
Specifically, because of the control process in the stoichiometric operation mode carried out in the period after it has begun for reducing NOx until the reduced-state recognizing means recognizes the above state, the reduction of NOx in the catalytic converter is basically completed after the dead time from the time when the reduced-state recognizing means recognizes the above state. Thereafter, when the reducing agent amount data generating means generates data representing an integrated amount of the reducing agent (HC, CO, H2, etc.) given to the catalytic converter via the exhaust gas, in the period after the control process in the stoichiometric operation mode has begun until the reduced-state recognizing means recognizes the above state, the generated data corresponds to the total amount of NOx absorbed by the catalytic converter during the execution of the control process in the lean operation mode prior to the execution of the control process in the stoichiometric operation mode. As the deterioration of the catalytic converter progresses, the total amount of NOx that can be absorbed thereby during the control mode in the lean operation mode is reduced. Therefore, the integrated amount of the reducing agent represented by the data generated by the reducing agent amount data generating means in the above period is correlated to the deteriorated state of the catalytic converter. It is thus possible to evaluate the deteriorated state of the catalytic converter based on the data generated by the reducing agent amount data generating means.
The amount of the reducing agent can be estimated from the amount of fuel supplied to the internal combustion engine and a command value for the amount of fuel to be supplied to the internal combustion engine.
The apparatus preferably further comprises absorption saturated-state recognizing means for recognizing whether the absorption of the nitrogen oxide by the catalytic converter is saturated or not while the control process in the stoichiometric operation mode is being executed by the control means, the catalytic converter deterioration evaluating means comprising means for evaluating the deteriorated state of the catalytic converter based on the data generated by the reducing agent amount data generating means while the control process in the stoichiometric operation mode is being executed, only when the control means switches from the control process in the lean operation mode to the control process in the stoichiometric operation mode after the absorption saturated-state recognizing means recognizes that the absorption of the nitrogen oxide by the catalytic converter is saturated.
When the control process in the lean operation mode is carried out until the absorption saturated-state recognizing means recognizes that the absorption of NOx by the catalytic converter is saturated, the total amount of NOx absorbed by the catalytic converter in the saturated state is the amount of NOx that can be absorbed to a maximum by the catalytic converter, and is distinctly correlated to the deteriorated state of the catalytic converter. Therefore, the total amount of NOx decreases monotonously as the deterioration of the catalytic converter progresses. When the control process in the stoichiometric operation mode for the reduction of NOx is carried out after the absorption of NOx by the catalytic converter is saturated, the reducing agent amount data generating means produces data representing an integrated amount of the reducing agent corresponding to the total amount of NOx in the saturated state. Depending on the operating conditions of the internal combustion engine, the control means may switch the control process in the lean operation mode to the control process in the stoichiometric operation mode before the absorption of NOx by the catalytic converter is saturated, i.e., when the catalytic converter can absorb more NOx.
The catalytic converter deterioration evaluating means evaluates the deteriorated state of the catalytic converter based on the data generated by the reducing agent amount data generating means while the control process in the stoichiometric operation mode is being executed, only when the control means switches from the control process in the lean operation mode to the control process in the stoichiometric operation mode after the absorption saturated-state recognizing means recognizes that the absorption of the nitrogen oxide by the catalytic converter is saturated.
In this manner, the integrated amount of the reducing agent represented by the data generated by the reducing agent amount data generating means corresponds to the total amount of NOx in the saturated state of the catalytic converter. Thus, the deteriorated state of the catalytic converter can appropriately be evaluated based on the above data.
With the absorption saturated-state recognizing means, the apparatus preferably further comprises nitrogen oxide amount data generating means for sequentially generating data representing an integrated amount of the nitrogen oxide given to the catalytic converter while the control process in the lean operation mode is being executed by the control means, the the absorption saturated-state recognizing means comprising means for determining whether the absorption of the nitrogen oxide by the catalytic converter is saturated or not by comparing the integrated amount of the nitrogen oxide represented by the data generated by the nitrogen oxide amount data generating means with a predetermined threshold value.
The predetermined threshold value to be compared with the integrated amount of the nitrogen oxide represented by the data generated by the nitrogen oxide amount data generating means is preferably established depending on a latest result of the deteriorated state of the catalytic converter evaluated by the catalytic converter deterioration evaluating means.
Specifically, the total amount of NOx absorbed by the catalytic converter while the absorption of NOx by the catalytic converter is being saturated varies depending on the deteriorated state of the catalytic converter, as described above. Therefore, by establishing the predetermined threshold value to be compared with the integrated amount of the nitrogen oxide depending on the latest evaluated result of the deteriorated state of the catalytic converter, it can properly be recognized that the absorption of NOx in the catalytic converter is saturated.
If the predetermined threshold value to be compared with the integrated amount of the nitrogen oxide is established depending on the latest evaluated result of the deteriorated state of the catalytic converter, then the control means preferably comprises means for canceling the control process in the lean operation mode and executing the control process in the stoichiometric operation mode when the absorption saturated-state recognizing means recognizes that the absorption of the nitrogen oxide by the catalytic converter is saturated while the control process in the lean operation mode is being executed.
When the absorption of NOx by the catalytic converter is saturated while the control process in the lean operation mode is being executed, the catalytic converter cannot absorb NOx unless the absorbed NOx is reduced. By establishing the predetermined threshold value depending on the latest evaluated result of the deteriorated state of the catalytic converter, at or nearly at the time when the absorption of NOx by the catalytic converter is actually saturated, the saturated state can be recognized by the absorption saturated-state recognizing means. Therefore, by canceling the control process in the lean operation mode and executing the control process in the stoichiometric operation mode depending on the recognition of the saturated state, excessive NOx that cannot be absorbed by the catalytic converter is prevented from passing through the catalytic converter and being discharged.
The estimating means comprises means for generating the data representing the estimated value of the output of the exhaust gas sensor according to an algorithm constructed based on a model of the exhaust system, which represents a behavior of the exhaust system regarded as a system for generating the output of the exhaust gas sensor from the air-fuel ratio of the exhaust gas entering the catalytic converter via a response delay element and a dead time element.
By determining a model which represents a behavior of the exhaust system in view of a response delay element and a dead time element of the exhaust system and performing the process of the estimating means according to an algorithm based on the model, the data representing the estimated value of the output of the exhaust gas sensor after the dead time of the exhaust system can properly be generated.
Specifically, the apparatus further comprises an air-fuel ratio sensor disposed upstream of the catalytic converter for detecting the air-fuel ratio of the exhaust gas entering the catalytic converter, the estimating means comprising means for generating the data representing the estimated value of the output of the exhaust gas sensor, using data of the output of the exhaust gas sensor and data of an output of the air-fuel ratio sensor.
Using data of the output of the air-fuel ratio sensor which corresponds to the detected value of an input to the exhaust system and data of the output of the exhaust gas sensor which corresponds to the detected value of an output to the exhaust system, highly reliable data can be generated as representing the estimated value of the output of the exhaust gas sensor after the dead time of the exhaust system. As a consequence, the reduced state of NOx in the stoichiometric operation mode can accurately be recognized based on the data representing the estimated value of the output of the exhaust gas sensor. Hence, it is possible to adequately determined whether to switch from the control process in the stoichiometric operation mode to the control process in the lean operation mode. As the reduced state of NOx can accurately be recognized, for evaluating the deteriorated state of the catalytic converter, it is possible to accurately generate data representing the integrated amount of the reducing agent required until the reduction of NOx is completed during the execution of the control process in the stoichiometric operation mode after the execution of the control process in the lean operation mode. Thus, the evaluated result of the deteriorated state of the catalytic converter based on the data representing the integrated amount of the reducing agent is made highly reliable.
According to the algorithm of the estimating means based on the model of the exhaust system, it may be possible to generate the data representing the estimated value of the output of the exhaust gas sensor, using data (e.g., a target value for the upstream-of-catalytic-converter air-fuel ratio) generated by the control means as defining the upstream-of-catalytic-converter air-fuel ratio in order to control the upstream-of-catalytic-converter air-fuel ratio in the control process in the stoichiometric operation mode, rather than the data of the output of the air-fuel ratio sensor. However, for increasing the accuracy of the data representing the estimated value of the output of the exhaust gas sensor, it is preferable to use the data of the output of the air-fuel ratio sensor which represents the actual input to the exhaust system.
For performing the process of the estimating means based on the model of the exhaust system, the model of the exhaust system has a parameter to be set to a certain value for defining its behavior. While the parameter may be of a predetermined fixed value, it is preferable to identify the parameter of the model sequentially on a real-time basis in order to achieve matching between the model and the actual behavior of the exhaust system. With the air-fuel sensor provided for detecting the upstream-of-catalytic-converter air-fuel ratio, the parameter of the model can be identified using the data of the output of the air-fuel sensor and the data of the output of the exhaust gas sensor.
According to the present invention, the apparatus further comprises identifying means for sequentially identifying the value of a parameter to be established of the model of the exhaust system, using the data of the output of the exhaust gas sensor and the data of the output of the air-fuel ratio sensor, while the control process in the stoichiometric operation mode is being executed by the control means, the estimating means comprising means for generating the data representing the estimated value of the output of the exhaust gas sensor, using the value of the parameter of the model which is identified by the identifying means, as well as the data of the output of the exhaust gas sensor and the data of the output of the air-fuel ratio sensor.
With the above arrangement, since the parameter of the model can sequentially be identified based on the actual behavior of the exhaust system, when the process of the estimating means is carried out using the parameter of the model as well as the data of the output of the exhaust gas sensor and the data of the output of the air-fuel ratio sensor, the accuracy of the estimated value of the output of the exhaust gas sensor represented by the data which is generated by the estimating means can be increased. As a result, the reduced state of NOx in the stoichiometric operation mode for the reduction of NOx can be recognized more accurately. Thus, it is possible to adequately determined whether to switch from the control process in the stoichiometric operation mode to the control process in the lean operation mode. With the deteriorated state of the catalytic converter being thus evaluated, the reliability of the deteriorated state of the catalytic converter can be increased.
Preferably, the parameter of the model which is identified by the identifying means includes a gain coefficient relative to the response delay element and a gain coefficient relative to the dead time element.
By identifying the gain coefficient relative to the response delay element and the gain coefficient relative to the dead time element as the parameter, proper matching can be achieved between the model and the behavior of the exhaust system, and hence the accuracy of the estimated value of the output of the exhaust gas sensor which is represented by the data generated by the estimating means according to the algorithm based on the model can be increased.
The model of the exhaust system preferably comprises a discrete-time system model which expresses the output of the exhaust gas sensor in each control cycle, using the output of the exhaust gas sensor in a past control cycle prior to the control cycle and the output of the air-fuel ratio sensor in a control cycle prior to the dead time of the exhaust system.
By thus constructing the model of the exhaust system as a discrete-time system model, the behavior of the exhaust system can appropriated by the model, and it is easy to construct the algorithm of the process of the identifying means and the process of the estimating means.
With the model of the exhaust system being constructed as a discrete-time system model, a coefficient relative to the output of the exhaust gas sensor and a coefficient relative to the output of the air-fuel ratio sensor in the model are provided as the parameter of the model. The coefficient relative to the output of the exhaust gas sensor becomes the gain coefficient relative to the response delay element, and the coefficient relative to the output of the air-fuel ratio sensor becomes the gain coefficient relative to the dead time element.
Preferably, the control process in the stoichiometric operation mode which is executed by the control means comprises a process for generating, according to a feedback control process, a manipulated variable which defines the air-fuel ratio of the exhaust gas entering the catalytic converter in order to converge the estimated value of the output of the exhaust gas sensor which is represented by the data generated by the estimating means to the target value, and manipulating the air-fuel ratio of an air-fuel mixture to be combusted by the internal combustion engine depending on the manipulated variable.
With the air-fuel ratio sensor provided, the control process in the stoichiometric operation mode which is executed by the control means comprises a process for generating, according to a first feedback control process, a target air-fuel ratio (a target air-fuel ratio for the upstream-of-catalytic-converter air-fuel ratio) for the exhaust gas entering the catalytic converter in order to converge the estimated value of the output of the exhaust gas sensor which is represented by the data generated by the estimating means to the target value, and manipulating, according to a second feedback control process, the air-fuel ratio of an air-fuel mixture to be combusted by the internal combustion engine in order to converge the air-fuel ratio detected by the air-fuel ratio sensor to the target air-fuel ratio.
In the control process in the stoichiometric operation mode, as described above, a manipulated variable which defines the upstream-of-catalytic-converter air-fuel ratio (a target air-fuel ratio for the upstream-of-catalytic-converter air-fuel ratio, a regulated amount for the fuel supply quantity of the internal combustion engine, etc.) is generated according to the feedback control process, and the air-fuel ratio of an air-fuel mixture combusted by the internal combustion engine is manipulated according to the manipulated variable, so that the upstream-of-catalytic-converter air-fuel ratio for converging the estimated value of the output of the exhaust gas sensor and hence the actual output of the exhaust gas sensor to their target value can appropriately be controlled.
With the air-fuel ratio sensor provided, the target air-fuel ration which is a target air-fuel ratio for the upstream-of-catalytic-converter air-fuel ratio is generated as the manipulated variable according to the first feedback control process, and the air-fuel ratio of the air-fuel mixture combusted by the internal combustion engine is manipulated according to the second feedback control process so as to converge the air-fuel ratio detected by the air-fuel ratio sensor to the target air-fuel ratio. In this fashion, the upstream-of-catalytic-converter air-fuel ratio can be controlled more reliably to converge the estimated value of the output of the exhaust gas sensor and hence the actual output of the exhaust gas sensor to their target value.
As a result, NOx can smoothly be reduced in the catalytic converter by executing the control process in the stoichiometric operation mode.
The feedback control process for generating the manipulated variable including the target air-fuel ratio for the exhaust gas entering the catalytic converter preferably comprises a sliding mode control process. Preferably, the sliding mode control process comprises an adaptive sliding mode control process.
The adaptive sliding mode control process is a combination of an ordinary sliding mode control process and a control law referred to as an adaptive law (adaptive algorithm) in order to minimize the effect of a disturbance or the like. More specifically, the sliding mode control process generally uses a function referred to as a switching function comprising the difference between a controlled variable (the output of the exhaust sensor) and its target value, and it is important to converge the switching function to xe2x80x9c0xe2x80x9d. The ordinary sliding control process uses a control law referred to as a reaching control law in order to converge the switching function to xe2x80x9c0xe2x80x9d. When subjected to the effect of a disturbance or the like, however, it is difficult for the reaching control law alone to achieve a sufficient level of stability and quick response with which to converge the value of the switching function to xe2x80x9c0xe2x80x9d. On the other hand, the adaptive sliding mode control process uses a control law referred to as an adaptive law (adaptive algorithm) in addition to the reaching control law order to converge the value of the switching function to xe2x80x9c0xe2x80x9d while minimizing the effect of a disturbance or the like.
By using the sliding mode control process, particularly, the adaptive sliding mode control process, for generating a manipulated variable such as the target air-fuel ratio, it is possible to generate a manipulated variable suitable for stably and quickly performing the control process of converging the output of the exhaust gas sensor to the target value. As a result, when the control process in the stoichiometric control mode for reducing NOx is performed after the control process in the lean operation mode has been carried out, NOx in the catalytic converter can be reduced quickly and smoothly. Consequently, the period in which to inhibit the control process in the lean operation mode for reducing NOx can be shortened, providing more opportunities for performing the control process in the lean operation mode.
Under the operating conditions for continuing the control process in the stoichiometric control mode, since the estimated value of the output of the exhaust gas sensor and hence the actual output of the exhaust gas sensor can be controlled at their target value highly stably with a quick response, the desired purifying capability of the catalytic converter can reliably be maintained.
With the air-fuel sensor provided and the control process in the stoichiometric control mode performed according to the first and second feedback control processes, the second feedback control process preferably comprises a control process carried out by a recursive-type feedback control means.
Specifically, the recursive-type feedback control means comprises an adaptive controller or an optimum regulator. By manipulating the air-fuel ratio of the air-fuel mixture combusted by the internal combustion engine to converge the air-fuel ratio (upstream-of-catalytic-converter air-fuel ratio) detected by the air-fuel ratio sensor to the target air-fuel ratio according to a control process of the recursive-type feedback control means, the upstream-of-catalytic-converter air-fuel ratio can be controlled at the target air-fuel ratio while quickly catching up dynamic changes such as changes in the operating conditions of the internal combustion engine and time-dependent characteristic changes of the internal combustion engine. Accordingly, the upstream-of-catalytic-converter air-fuel ratio can be controlled with a highly quick response to converge the output of the exhaust gas sensor to the target value.
The recursive-type feedback control means determines a new feedback manipulated variable according to a recursive formula which contains a predetermined number of time-series data prior to the present time of a feedback manipulated variable for the air-fuel ratio of the air-fuel mixture combusted by the internal combustion engine, e.g., a corrective quantity for the fuel supply quantity. The recursive-type feedback control means should preferably comprise an adaptive controller.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.